An operating environment within a turbine is typically thermally and chemically hostile to materials used to form turbines and turbine components. Significant advances in high temperature alloys for these turbine components have been achieved through the formulation of superalloy materials, including iron-based, nickel-based, nickel-iron-based and cobalt-based superalloys. Turbine components formed from these materials, however, normally cannot withstand long service exposures at high temperatures if located in certain sections of a turbine.
One proposed solution to permit turbine components to withstand long-service, high-temperature exposures includes protecting surfaces of turbine components with a coating. The coating may comprise an aluminide coating or a thermal barrier coating system (TBCS). A TBCS includes an environmentally-resistant bond coat on the component (substrate) and a layer of thermal-insulating ceramic layer as a thermal barrier coating (TBC), which is applied over the bond coat.
A TBC typically comprises a layer that includes, but is not limited to, zirconia (ZrO.sub.2) in the form of partially- or fully-stabilized in the cubic zirconia crystal form. This layer should reduce generation of phase transformation stresses. The partially- or fully-stabilized zirconia (hereinafter referred to as "modified zirconia") is stabilized by one of yttria (Y.sub.2 O.sub.3), magnesia (MgO), and other oxides. Modified zirconia exhibits low thermal conductivity, and thus provides thermal protection for turbine components. Modified zirconia possesses a thermal conductivity that is generally about 1/20.sup.th that of a turbine component's substrate, so as to provide a temperature drop across the TBC in a range between about 25.degree. C. and about 150.degree. C., depending on the TBC layer's thickness and a substrate's cooling rate. Further, modified zirconia is a desirable material for thermal protection applications of turbine components because the modified zirconia reasonably adheres well to turbine substrate materials, compared to the adherence of other ceramic materials.
Bond coats for a TBCS typically comprise an oxidation-resistant alloy, such as MCrAlY, where M is at least one of iron, cobalt and nickel. Alternatively, a bond coat comprises a diffusion aluminide or platinum-aluminide material that forms an oxidation-resistant intermetallic component. Bond coats mechanically interlock, or join, the substrate to the TBC. Bond coats also protect the underlying turbine component substrate by forming a protective oxidation barrier that protects the turbine component substrate when a TBC spalls, separates from the turbine component, and exposes the turbine component substrate to hot gases. The protective, oxidation barrier is typically formed by reaction of oxygen with a bond coat's constituents.
A bond coat's aluminum content creates a desirable, strongly-adherent, continuous aluminum oxide layer. The desirable aluminum oxide layer acts as a protective oxidation barrier, and typically comprises an alumina scale, which is often referred to as a "thermally-grown oxide" (TGO). The alumina scale is slowly formed at elevated temperatures, and protects bond coats from rapid oxidation and hot corrosion during elevated temperature applications.
Oxidation that occurs to form the TGO during elevated temperature service gradually depletes aluminum from the bond coat. Eventually, the bond coat's aluminum content is depleted to a point that the protective TGO growth cannot occur. This reduced, aluminum level in a bond coat is undesirable because, in addition to the non-growth of protective TGOs, a growth of undesirable, non-protective oxides is promoted.
The non-protective oxides promote spallation in the TBCS. The spallation may cause separation of the TBCS components during elevated temperature applications. Separation at interfaces between the bond coat and TBC due to expansion amount mismatches occurs. These expansion amount differences are mainly due to different coefficients of thermal expansion (CTE) of the components of the TBCS. Therefore, a need for a bond coat in which non-protective oxides growth is minimal, and spallation and separation are avoided, exists.
Bond coats comprising layered, step-graded thermal properties have been proposed to reduce non-protective oxides growth, and to avoid spallation and separation. These layered, step-graded bond coats attempt to avoid spallation and separation by minimizing differences in thermal expansion in thermal barrier coating system (TBCS) components. FIG. 1 illustrates an example of one thermal barrier coating system 100, where a bond coat 114 includes distinct, step-graded inner and outer layers 111 and 113. Inner and outer layers 111 and 113 possess CTE values that are between a CTE value of the substrate 110 and the CTE of the ceramic TBC 112, respectively. The CTE value of the inner layer 111 is closer to the CTE value of the substrate 110 than to the CTE value of the TBC 112. The CTE value of the outer layer 113 is closer to the CTE value of the ceramic 112 than to the CTE value of the substrate 110. Thus, the expansion amounts, under similar conditions, of inner layer 111 and its adjoining substrate 110, and outer layer 113 and the TBC 112 are relatively close in value. Thus, expansion amount differences are minimal, under similar conditions, and spallation and separation at their respective interfaces, 117 and 118, are avoided. An interface 115 between the inner layer 111 and the outer layer 113, however exhibits abrupt, stepped, and discontinuous changes in material property characteristics (hereinafter referred to as "material characteristics"), including CTEs. Therefore, expansion amounts at the interface 115 may differ in amounts that cause spallation and separation thereat. Accordingly, a TBCS may have its overall TBCS life shortened.
The widths of the different layers that are illustrated in FIG. 1 schematically correspond to the expansion that each layer would undergo if each layer were unconstrained at its interfaces with its neighbors as graphically illustrated, steps in expansion amounts are evident at interfaces 1, 2, and 3. The expansion at interfaces 1, 2, and 3 are not similar and the differences there at are so great that separation at the interfaces often causes failure of the TBCS.
Therefore, a need for a bond coat for a thermal barrier coating system that avoids spallation and separation exists. The bond coat should exhibit material characteristics, such as CTEs, that minimize stresses and separation.